This is the background image for an Adobe Acrobat Capture page with image plus hidden text.

This is created by the application that created the file (Adobe Acrobat Capture, most likely), as alternative text for the scanned image.

You can probably remove it using the Sanitize Document command of Acrobat, although a better option is to look under the preferences of Capture itself (if you're the one creating these files) and disable it there.